1. Technical Field
The invention relates to a control circuit of a driving circuit of a discharge lamp.
2. Description of the Related Art
Fluorescent lamps are typically composed of a glass tube which contains a small quantity of mercury, a low pressure inert gas and phosphorous powders which coat the inside part of the tube. At the extremities two electrodes are present which, connected to a suitable driving circuit, form the arc that permits the discharge of the gas to be generated and maintained.
Among the possible driving circuits the so-called high frequency ballast circuits can be enumerated: these are circuits at whose output an alternating voltage signal is generated at a frequency and amplitude necessary to keep the lamp on; this waveform is produced by a circuit that comprises a couple of transistors that switch at a frequency of tens of KHz, a current limiting coil and a filtering capacitance.
A typical ballast circuit is described in FIG. 1 where a half-bridge circuit, which comprises the transistors T1 and T2 and is arranged between an high voltage Vh and ground GND, drives a lamp LP by means of a voltage Vin. The circuit in FIG. 1 shows an inductance L for limiting the current which is arranged between the common terminal of the transistor T1 and T2 and the lamp LP, a filter capacitor C arranged in parallel to the lamp LP and a capacitor Clp arranged between the lamp LP and ground GND. A driving circuit 1 drives the half bridge so that a voltage Vin is across the lamp LP and the inductance L and a voltage Vout is across the capacitor C. The lamp LP can be considered an “open circuit” before the ignition and as a resistor when the lamp is turned on.
The frequency response of the circuit in FIG. 1 is shown in FIG. 2. The lamp ignition is achieved in three phases each one of which corresponds to an operative point on the waveform in FIG. 2.
In a first phase, that is a preheating phase, the lamp is off and the corresponding point in the waveform in FIG. 2 is a point of the line A. During the first phase the resonant circuit is driven with a high frequency signal for preheating the electrodes of the lamp; the current and the voltage across the lamp are at the minimum root-mean square value. The half bridge drives a load of inductive type because the operative point is at a frequency higher than the frequency fr(HighQ) on the diagram in FIG. 2.
In a second phase, that is the ignition phase, the switching frequency of the transistors of the half bridge is decreased by providing an increase of the current and of the voltage of the lamp. Such an increase occurs to reach the ignition voltage of the gas which allows the ignition of the lamp. During this phase, the operative point goes towards fr(HighQ) on the line A in the diagram in FIG. 2: the half bridge drives a load of the inductive type if the ignition of the lamp occurs before the frequency reaches the value fr(HighQ). The situation wherein the lamp ignites close to the resonance frequency fr(HighQ) leads to a condition wherein the voltage and current assume too high values which could cause the destruction of the transistors of the half bridge. This is possible in the case of the ageing of the lamp which causes the increase of the ignition voltage of the gas with corresponding decrease of the frequency value corresponding to the ignition.
In the third phase, the run/burn phase or run phase, the load driven by the half bridge will be of the inductive type but with a resistor (the lamp) in series with the capacitor C and the inductor L. The operative point is on the line B of the diagram in FIG. 2 and it is characterized by the minimum frequency value and by voltage/current values of the lamp which are lower than ignition values and higher than the preheating values. In this case the correct operating condition is that where the load is a load of the inductive type, that is the operating frequency remains above the new resonance frequency fr(LowQ). If the lamp is damaged so that the operating frequency is too close to the resonance frequency fr(LowQ), the reached voltage and current high values could bring to the destruction of the transistors of the half bridge. Also, since the lamp in this phase is turned on, a sudden variation of the resistive value of the lamp could cause an increase of the quality factor Q with a movement of the operating point on a line C of the diagram in FIG. 2 (capacitive performance) and determining a condition of “hard switching” which is very dangerous for the half bridge.